In digital communication, as a modulation and demodulation method for efficiently transmitting and receiving data, the quadrature amplitude modulation (QAM) method in which both phase information and amplitude information is used for data discrimination is known. At present, with the increase of the demand of a large capacity wireless communication system, it is required to increase the modulation multi-level number.
However, when the modulation multi-level number is increased, a problem in which a transmission error rate due to noise increases and noise immunity decreases occurs. In particular, a phase noise generated by a reference oscillator (Local Oscillator; LO) provided in a transmission device or a reception device increases indeterminacy of phase information and deteriorates a bit error rate (BER) characteristic. Accordingly, for example, in order to perform highly reliable data communication by using a multi-level QAM method in which 256 or more signal points are used, a phase error produced by the phase noise needs to be compensated for at high accuracy. Further, at the same time, immunity to error due to other factors such as thermal noise and the like needs to be improved.
FIG. 14 is a block diagram showing a configuration of a demodulation device according to a related technology in a reception device of a digital wireless device system. Referring to FIG. 14, the demodulation device includes a reference oscillator 121, a detector 122, an analog/digital (A/D) converter 123, a carrier wave reproduction phase lock loop (Phase Lock Loop; PLL) in which a phase rotator 124, a phase error detector 125, a loop filter 126, and a numerical control oscillator 127 are connected in a loop, a QAM symbol demapping unit 128 which converts a received symbol into a bit string, and an error correction decoder 129.
The reference oscillator 121 outputs a reference signal having a predetermined fixed frequency. The detector 122 performs quadrature detection of an input signal by using the reference signal and generates an Ich (In-phase channel) baseband signal and a Qch (Quadrate-phase Channel) baseband signal. The generated baseband signal is converted into a digital signal through the A/D converter 123.
The phase rotator 124 corrects the phase error by rotating the phase of the received symbol that corresponds to the Ich digitalized baseband signal and the Qch digitalized baseband signal according to output information of the numerical control oscillator 127. The output signal of the phase rotator 124 is inputted to the phase error detector 125. The phase error detector 125 detects the phase error which remains in the received symbol and outputs it to the loop filter 126. The loop filter 126 removes an unnecessary high frequency component included in the phase error and outputs it to the numerical control oscillator 127. The numerical control oscillator 127 generates phase error information which specifies a phase rotation amount in the phase rotator 124 from the output of the loop filter 126 and outputs it.
As described above, by the operation of the carrier wave reproduction PLL in which the phase rotator 124, the phase error detector 125, the loop filter 126, and the numerical control oscillators 127 are connected in a loop, a stable phase locked state can be realized. Whereby, the phase error can be compensated for.
The received symbol to which the phase noise correction is performed by the phase rotator 124 is inputted to the phase error detector 125 and also, inputted to the QAM symbol demapping unit 128. The QAM symbol demapping unit 128 calculates a received bit string corresponding to the received symbol from the received symbol and the error correction decoder 129 performs an error correction process and outputs the received bit string. When a soft decision decoder which receives likelihood information indicating a certainty of each received bit and performs the correction process is used for the error correction decoder 129, the QAM symbol demapping unit 128 outputs the bit string in which the likelihood information is reflected. For example, the QAM symbol demapping unit 128 which outputs the bit string in which the likelihood information is reflected is described in patent literature 1.
As described above, the demodulation device according to the related technology compensates for the phase error by the carrier wave reproduction PLL and realizes improvement in error immunity by the error correction process performed in a later stage. However, there is a case in which when the accuracy of the phase error detector 125 decreases by the phase noise included in the baseband signal outputted by the detector 122, a thermal noise, and the like, the satisfactory BER performance cannot be obtained. In this case, a technology in which the phase error compensation accuracy is improved by adaptively adjusting a bandwidth of the loop filter 126 in the carrier wave reproduction PLL is disclosed in patent literature 2, patent literature 3, and patent literature 4. However, there is a case in which even when these technologies are used, a sufficient effect cannot be obtained.
As a method other than the method using the carrier wave reproduction PLL, there is a method in which a known signal (a pilot signal) is inserted in the transmission signal and the phase error is compensated for by using this known signal. FIG. 15 is a block diagram showing a configuration of the demodulation device using this method. Referring to FIG. 15, the demodulation device includes a reference oscillator 131, a detector 132, an A/D converter 133, a QAM symbol demapping unit 137, and an error correction decoder 138 like the demodulation device shown in FIG. 14. Further, the demodulation device includes an interpolation filter 135 which estimates the phase error between the received pilot symbols from the received pilot symbol corresponding to the pilot signal, a delay circuit 134 which delays a signal by the number of symbols corresponding to a delay incurred by the interpolation process, and a phase rotator 136 which corrects the estimated phase error.
The demodulation device shown in FIG. 15 performs quadrature detection of the input signal in the detector 132 by using the reference signal with the fixed frequency outputted by the reference oscillator 131 and generates the Ich (In-phase channel) baseband signal and the Qch (Quadrate-phase Channel) baseband signal like the demodulation device shown in FIG. 14. The generated baseband signal is converted into the digital signal through the A/D converter 133.
The received symbol that corresponds to the Ich digitalized baseband signal and the Qch digitalized baseband signal is inputted to the delay circuit 134. Here, only when the received symbol is the pilot symbol corresponding to the known pilot signal, the received symbol is also inputted to the interpolation filter 135. The interpolation filter 135 estimates the phase error in the received symbol between the pilot symbols from a plurality of the pilot symbols by the interpolation process. The phase rotator 136 rotates the phase of the received symbol based on the phase error information outputted by the interpolation filter and corrects the phase error in the received symbol.
The received symbol to which the phase error is compensated for by the interpolation filter 135 is inputted to the QAM symbol demapping unit 137 like the demodulation method performed by the demodulation device shown in FIG. 14 and converted into the (soft decision) received bit string. After this process, the error correction process is performed in the error correction decoder 138 and it is outputted as output data.
For example, the demodulation method using the interpolation filter which uses the pilot symbol is described in non-patent literature 1 and non-patent literature 2.